The present invention relates to a magnetic disk drive and, in particular, it relates to a servo controller for positioning a magnetic head at a target track.
On a magnetic disk mounted on a magnetic disk drive, a plurality of data tracks are formed concentrically and servo data is recorded in a radial direction of the disk in advance. The servo data is comprised of positional information such as track data, sector data and burst signals. The track data is information indicating track addresses and the sector data is information indicating sector numbers and, based on the track data read by a magnetic head, an approximate position of the magnetic head or, in other words, the data track where the magnetic head is located can be determined. Further, burst signals are comprised of a plurality of burst pattern rows, each of which has data recording areas arranged at regular intervals in the radial direction of the disk and having different phases from each other and, based on signals output from the magnetic head according to the burst pattern, an accurate position of the magnetic head, or, in other words, a deviation of the position of the magnetic head in the inner or outer side from the data track where the magnetic head is located can be detected.
Information is read from or written to the magnetic disk after the magnetic head is moved to be located at a target data track while checking the approximate position of the magnetic head based on the track data read by the magnetic head and, then, the magnetic head is positioned at the target data track accurately based on the signals output from the magnetic head according to the burst pattern, during which the magnetic disk rotates. Further, even while the information is read or written, the magnetic head is feedback-controlled so that it is positioned at a constant position with respect to the target track based on the signals output from the magnetic head according to the burst pattern.
As the data track width is reduced for high-density recording, repeatable runout (RRO) errors, which may occur because the servo data recorded on the disk is not perfect concentric circles, become a problem. In order to compensate for the RRO, a method for recording RRO compensation data at the end of servo data areas recorded on the disk as set forth in Patent Document 1 (Japanese Unexamined Patent Publication 2002-525776) is known. Further, in Patent Document 2 (Japanese Unexamined Patent Publication 2003-505818), a specific method for obtaining the RRO compensation data by measuring low frequency components is set forth.